Butte, Montana Copper Ores Illustrating Replacement Textures

Pyrite (Py; yellow) partially replaced by chalcocite (Cc; bluish grey). The Butte, Montana copper ores illustrate typical replacement textures: 1) irregularly shaped replacement remnants ("sea islands") of pyrite being replaced by chalcocite, 2) caries texture in which chalcocite embayments into pyrite are concave with respect to the host pyrite, and 3) vein texture in which chalcocite replacement veins traverse pyrite. Holes in the polished section are black. Ore microscopy, reflected light, low magnification.



Bornite (Bo; blue) patches in chalcocite (Cc; grey) in Butte, Montana copper ores. The bornite areas may represent irregularly shaped replacement remnants ("sea islands") of bornite being replaced by chalcocite. However, the smoothly rounded character of the margins of the bornite areas suggests that they more likely have formed by exsolution out of the chalcocite. Bornite and chalcocite form a continuous solid solution series that may deposit a single solid at high temperature that can undergo exsolution during subsequent declining temperatures. Freshly polished bornite is tan in reflected light, but it quickly (few days or weeks) becomes blue, and then purple by oxidation in air. Ore microscopy, reflected light, medium magnification.



A single grain of tetrahedrite (Td; grey) surrounded and partly replaced by bornite (Bo; tan) and chalcocite (Cc; bluish grey). The tetrahedrite grain exhibits and excellent caries texture, along it left side, in which chalcocite has formed a ragged replacement embayment into the tetrahedrite. Bornite occurs as abundant irregularly shaped replacement remnants ("sea islands") that are being replaced by chalcocite. This polished section has been freshly polished and therefore the bornite is still tan in color. Butte, Montana copper ores. Ore microscopy, reflected light, medium magnification.



Ore microscopic view of polished section where a sequence of replacement reactions define the sulfide paragenetic sequence. Early pyrite (Py; yellow) was replaced by subsequent chalcopyrite (Cp; deep yellow). Chalcopyrite was then replaced by bornite (Bo; purple) and bornite was finally replaced by chalcocite (Cc; bluish grey). Each of the replacements are shown by the presence of irregularly shape inclusions (sea islands) of the earlier mineral engulfed by the later mineral, and by the local presence of caries and vein textures. Thus, the paragenetic sequence of minerals deposited, as shown in this photomicrograph, is: pyrite (earliest) - chalcopyrite - bornite - chalcocite (latest). Butte, Montana copper ores. Ore microscopy, reflected light, high magnification.



Local open space filling texture in lead-zinc ores from the peripheral zone at Butte. Euhedral sphalerite (Sl; grey) was deposited early and followed by galena (Gn; white). The sphalerite crystal marked by "Sl" provides an excellent example of a rotund crystal shape that is typical of euhedral sphalerite crystals deposited in open space. A second rotund sphalerite crystal occurs to the left of the first crystal. A single crystal of quartz occurs at the contact between sphalerite and galena. Although it might be interpreted that the quartz had been deposited on sphalerite before galena, it is more likely that the quartz preferentially replaced galena after both sphalerite and galena had been deposited. Two NW-trending scratches tend to be broader in the softer galena than in the harder sphalerite. Butte, Montana copper ores. Ore microscopy, reflected light, medium magnification.



Very small blebs of chalcoprite (Cp; yellow) distributed through sphalerite (Sl; grey) in lead-zinc ores from the peripheral zone at Butte. This texture has usually be referred by ore microscopists as emulsion texture, and it has been interpreted to have formed by solid state exsolution of a solid solution during declining temperatures. The fact that such textures are confined to higher temperature ores (they are lacking in Tri-State ores) has seemed to support this interpretation for many ores. However, recently it has been clearly demonstated that some (or most) emulsion-like textures have developed by replacement in which copper has been introduced in later ore fluids and combined with the iron in iron-rich bands of the host sphalerite to form chalcopyrite blebs. It has been further demonstrated that the chalcopyrite blebs typically are restricted to the iron-rich bands. The chalcopyrite blebs are 1-5 µm across. Small holes in the polished surface are black. Butte, Montana copper ores. Ore microscopy, reflected light, high magnification.



Abundant covellite (Cv) and minor chalcocite (Cc) in Butte, Montana ores. Covellite exhibits a variety of colors due to its extreme reflective pleochoism. Depending upon its crystal orientation, covellite may be deep blue, medium blue, or light bluish grey. Although part of the white material marked Cc is chalcocite, much of it is covellite with an orientation that gives it a color and reflectance that nearly matches that of chalcocite. Rotation of the microscope stage shows that part of the white areas change to deep blue indicating that they are areas of covellite. Ore microscopy, reflected light, medium magnification.